Simplify path handling.

There will always be a single <path> element, so no need to handle enter, update
and exit cases.  WebKit seems to erroneously warn (in the error console) that it
can't parse `d=""`.  I can't see anything in the SVG spec disallowing an empty
`d` attribute so I think this is safe to ignore.

examples/spline/spline.js

@@ -28,18 +28,13 @@ var line = d3.svg.line()
     .x(function(d) { return d[0]; })
     .y(function(d) { return d[1]; });
 
+var path = vis.append("svg:path")
+  .data([points])
+    .attr("class", "line");
+
 function update() {
-  var path = vis.selectAll("path")
-    .data(points.length ? [points] : []);
-
-  path.enter().append("svg:path")
-      .attr("class", "line")
-      .attr("d", line);
-
   path.attr("d", line);
 
-  path.exit().remove();
-
   var circle = vis.selectAll("circle")
       .data(points, function(d) { return d; });